Last year it was revealed that the NHS had failed to investigate an astonishing number of unexpected deaths: more than 700 within a single trust. Sara Ryans son was one of them

Sara Ryan is a sharp, funny woman with short spiky hair and an acute bullshit detector. She used to laugh all the time, as did her son, Connor Sparrowhawk. She and her husband, Richard Huggins (Connors stepfather, who brought him up from the age of three), sit in the kitchen of their home in Oxford, the table heaped with biscuits and chocolate, and tell me stories about the son they nicknamed Laughing Boy.

Connor was epileptic, autistic and had learning disabilities. He was also very funny sometimes knowingly, sometimes not. The thing about Connor, his parents say, is he had no filter. There was the time they went camping, with Connor and his siblings. One night, the people who were camping next to us came around. And the older man had a beautiful, much younger girlfriend. Connor spent the whole evening talking about paedophiles. We all tried to smile it off, but when we got up in the morning theyd gone.

He could get things very wrong. Take the time they were staying on a farm and he became convinced the farmer was running an international crime syndicate. He decided he was wanted by Interpol. Hed march round, raging about the farmer being a wanted criminal. How did the farmer react? Oh, they were bezzy mates by the end, werent they? Sara says.

He had a strong sense of injustice, Richard adds, such faith in the law. He would rant and rave about how we were infringing his human rights. Hed always be slapping injunctions on us. What about? Oh, you know, Sara says, trying to keep a straight face, cleaning, washing up.

Three years ago, aged 18, Connor drowned in a bath at Slade House, a residential unit run by Southern Health, an NHS foundation trust. His parents had brought him there a few months before, after he became aggressive and violent, and they found themselves unable to cope. It felt as if we were buying a bit of time for everyone, including Connor, Richard says. These guys were professionals, Connor would get some support. We thought it was a terrible thing to have to do, but it was fair. Within a few days we thought the place wasnt very good. But we never, ever thought he wouldnt come out.

In October last year, a jury delivered a damning verdict, that serious failings and neglect had contributed to Connors death. Two months later, an astonishing report by audit firm Mazars, commissioned by NHS England in 2014 at the request of Connors family, found that Southern Health had failed to properly investigate the deaths of more than 700 people with learning disabilities or mental health problems, over a four-year period, from 2011 to 2015.

In a statement to the Commons on 10 December last year, health secretary Jeremy Hunt said that the Mazars report had raised serious concerns about Southern Health, which cares for about 45,000 people in Hampshire, Dorset, Wiltshire, Oxfordshire and Buckinghamshire. Our hearts go out to the families of those affected, he said. More than anything, they want to know that the NHS learns from tragedies such as what happened to Connor Sparrowhawk, and that is something we patently fail to do on too many occasions There is an urgent need to improve the investigation of, and learning from, the estimated 200 avoidable deaths we have every week across the system.

Connor is still everywhere in the family home. His paintings are on the wall, there are photographs of him and his siblings on the mantelpiece, literature about the case is stacked in piles. The night Connor died, Sara says, the house became more a campaigning centre than a home.

But once he went into Slade House, the blogs became more serious, highlighting perceived inadequacies in Connors care. Sara gave the staff nicknames; one psychiatrist became Dr Crapshite.

After almost four months at the unit, Connor seemed calmer and was looking forward to returning home for good (a meeting had been planned to discuss this). Richard asked to visit him on the evening of 3 July 2013 (Connor had to consent, because he was 18). He was very excited, gabbling, so I said, Should I come round and see you? And he was like, Yeah, yeah, yeah, yeah. But when Connor handed the phone back to the nurse, she said no, Connor didnt want him to come. He is still exasperated talking about it today. I said, I can hear him saying yes, Im coming round and thankfully I did. Because that was the last time I saw him.

On Connors 107th day in care, Sara received a call from a psychiatrist at the unit. She just said, Oh hello, are you at work? Its Connor, we found him unconscious in the bath. Hes on his way to the John Radcliffe hospital, well ring you if theres any change. Can you make your way there?

Sara says the psychiatrist was so casual, she didnt think there was anything to worry about; after all, Connor had been taken to hospital a few times after epileptic seizures. I was texting a couple of mates in the cab. I didnt even ring you, did I? she asks Richard. Then she began to worry and rang the unit back. I said, Was he breathing, was he conscious when he left? and they said, Oh, they got him breathing.

In September 2013, six weeks after Connor died, the Care Quality Commission visited Slade House, and failed it on all 10 counts it inspected: there was no battery in the defibrillator, no oxygen in the oxygen tank, no therapeutic interactions, there were traces of faeces in the furniture, medicines were out of date and on it went. Three months later, in December 2013, the Statt unit was closed down. In May 2014, it was reported that the CQC had returned to Slade House and that its services had improved; baths, however, were now banned in favour of showers. The reason was never revealed to Connors family. (The CQCs current rating for Southern Health is requires improvement.)

As soon as Connor died, people advised Sara and Richard to get in touch with the charity Inquest, to get hold of Connors records from Southern Health, to make sure the postmortem had been conducted correctly. We were given a list of about 15 things and told youve got to do these this morning or youll be screwed, and it was true we would have been.

With a strong social support group, they were in a relatively good position to find out what had happened to their son but even so it almost did for them. There have been times, God knows, when both of us have thought, what is the point of carrying on? Richard says. Ive got high blood pressure and am on antidepressants. Ive lost part of my job. Deborah Coles, director of Inquest, has worked with the family since Connors death. Without their determination, she says, the truth would have been hidden from public scrutiny. What about all the Connors who dont have families to speak out for them?

Look beyond the stark headline numbers of 2015s Mazars report and the detail is just as shocking. Of 10,306 deaths of service users between April 2011 and March 2015, 722 were categorised as unexpected; of these only 30% were investigated. Sixty-four per cent of investigations did not involve the family. Most shocking of all, less than 1% of deaths in learning disability services were investigated (compared with 60% of unexpected deaths in adult mental health services). Southern Health came in for severe criticism: The failure to bring about a sustained improvement in the identification of unexpected death and in the quality and timeliness of reports into those deaths is a failure of leadership and government.

In December, Sara called for the chief executive of Southern Health, Katrina Percy, to resign. The trust acknowledged the failings documented in the Mazars report: We fully accept that our processes for reporting and investigating deaths of people with learning disabilities and mental health needs were not always as good as they should have been. But it also defended its record, stating that, National data on mortality rates confirms that the Trust is not an outlier and its rate of investigations is in line with that of other NHS organisations. (As if that makes it all right! Sara says.)

Percy made a public apology: Connor needed our support. We did not keep him safe and his death was preventable. She added that many changes had been made since Connor died. Percy is still in post.

For Sara, the Mazars report crystallised what she had always suspected. Its a eugenics thing, she says. Theres no value attached to their lives.

Bess the jack russell doesnt so much bark this time as howl.

Look, Richard says, they never set out to cause a commotion. If there had been a very different approach from day one, if thered been a culture of saying, lets sit down and see how we can sort this out, none of this would have happened. We werent confrontational from the beginning.

I always was, Sara corrects him.

No, Richard says quietly. We werent confrontational, we just wanted stuff done for Connor. We werent fighting just to fight.

Sara says it was an amazing moment, hearing Connor and Mazars and Southern Health being debated in the Commons at the end of last year. Now she believes many of the professionals involved have regrets for the wrong reasons. People have said to me, if Southern Health had just behaved a bit better, none of this would have come out, as if its a bit more problematic than its worth. I think a lot of people in health and social care still think, I dont want to lose my job over a bunch of learning disabled people, thank you very much. And yet Connor was better than all of us in many ways his generosity of spirit. No guile, no deceit, no lying, no avarice. He didnt want anything; he just wanted to be.

As well as a class action, Sara is looking at the possibility of bringing a corporate manslaughter charge against Southern Health. After Connors death, she was told that the trust could not be charged with this because the bar for gross negligence had not been met. Now, lawyers are taking another look. Were arguing that it was met, because of the earlier death in the bath.

In December, Jeremy Hunt promised a study into mortality rates of people with learning disabilities across the NHS, and to publish the number of avoidable deaths within each NHS trust, from this year. But as yet there has been no confirmation that there will even be an investigation into the 700-plus deaths identified by the Mazars report. Charlotte Haworth Hird, a lawyer representing Connors family, says: It is no good simply saying that investigation procedures will be different in future, if indeed they will. There needs to be effective investigation of those deaths that have already happened, and those responsible need to be held to account.

Three years on, Sara and Richard have called for a much wider public inquiry, into all those avoidable deaths. Its hard to know where they get the energy from. Sara says shes a changed person and not in a way that she likes.

Rage is quite new to me. Id have to be pushed into a rage before, she says. I was hugely optimistic.

But you always had a huge sense of justice, Richard says, a bit like Connor. A sense of fairness.

She nods. Yes, she says, but this is different. Rage is my daily business now.